Add custom backup messages

Hello,

A friend of mine uses your server tools but was disappointed about the lack of custom messages, so I went ahead and added what I thought was needed below. I'm guessing my formatting and bad style (lack of comments, committing to master branch, etc) won't please you but I thought I'd submit the changed code to you rather than just submitting an issue.

If you want to turn down the request due to my bad style, please let me know and I'll do it properly and submit again. These changes are pretty simple but, unless I'm missing something (don't know much about MC plugins) I believe they are correct.

Please consider this input!

Thank you,
jnickg
Should be
```java
backupWarningMessage = prop.getString();
```